(a) Field of the Invention
This invention relates to tongs used, for example, in the drilling industry for connecting and disconnecting lengths of pipe, e.g. drill pipe, and more particularly to an attachment for such a tong incorporating means to measure the torque being applied through the tong when a force is applied to the tong arm by means of a cable or rope.
(B) Description of the Prior Art
By definition, the torque applied through a tong is the product of the force applied, i.e. the tension in the line, and the moment arm, which is the perpendicular distance between the center of the pipe being connected and the line of action of the force It is known to incorporate a tension indicator in the cable to indicate the force being applied to the tong arm. However, because of the change of position of the tong arm as the pipe is turned, the length of the moment arm as above defined changes, being equal to the length of the connection line, i.e. the line from the center of the pipe to the point of connection of the cable to the tong arm only when such connection line is perpendicular to the cable. In general, the moment arm is equal to such distance multiplied by the sine of the angle (X) between the cable and the connection line.
It is desirable to provide a device overcoming the problem of the variation of the angle (X), whereby torque is correctly indicated by a force measuring device regardless of the angle (X). A search of certain areas of classification of United States patents revealed the following U.S. patents as relevant to the problem:
U.s. pat. No. 2,183,633 -- Zimmermann, PA1 U.s. pat. No. 2,801,539 -- Swenson, PA1 U.s. pat. No. 3,589,179 -- Nicolau, PA1 U.s. pat. No. 3,693,727 -- Bell.
The Zimmerman patent discloses a wrench employing two arms, both to be positioned with one end centered on a nut to be turned but extending in different directions, with a force indicator therebetweeen perpendicular to one arm. Force applied at any angle to the other arm is transmitted perpendicularly through the force indicator to the one arm. However, as the spring type force indicator employed shortens with application of force, the force indicator is no longer precisely perpendicular to the one arm so that the force indicator scale needs to be nonlinear in order to indicate torque.
The Swenson patent shows pipe tongs and cable with a force measuring unit in the cable. The cable end connected to the tongs passes over an arcuate support mounted on the tongs, whereby the angle between the cable and the connection line is constant, so that a torque gage connected to the force measuring unit can be linearly calibrated. The angle through which the tongs can be turned while the torque gage gives a correct reading is limited by the length of the arcuate support.
The Nicolau patent shows a pipe tong to be actuated by a cable and incorporating a torque indicator which gives a true torque indication regardless of the cable's angle to the tong arm. This is achieved by passing the cable through an eye in the torque arm, the end of the cable then extending perpendicular to the arm to a force measuring unit, so that only the component of cable tension perpendicular to the arm is applied to the force indicator. To compensate for the reaction of the tong at its eye where the cable passes through, the force measurer is mounted so that the torque effective component of the reaction force is also applied to the force measuring unit. The mounting of the force measuring unit includes relatively movable yoke and U shaped support members which must remain free to move in order that the torque indicator will give correct readings.
The Bell patent shows a pipe tong to which force is applied by a cable and which includes a torque indicator. To overcome the problem of variable angle between cable and tong, a special connection between them is employed. A lever is pivotally mounted on the tong arm. The cable is attached to one end of the lever. A load cell is positioned between the other end of the lever and the tong with the load cell axis perpendicular to the lever arm. It is said that the load cell measures the true torque applied to the lever and since the tong torque must be equal and opposite, the load cell measures the true torque applied to the tong arm.
It will be seen that all of the foregoing U.S. patents which are intended to give a linear indication of torque involve a mounting including some form of relatively moving parts, i.e. a sector on which the cable winds (Swenson), relatively slidable yoke and U shaped members (Nicolau), and a pivotally mounted lever (Bell).
It is an object of the present invention to provide a torque indicator for attachment to a tong arm that will give a correct reading independent of the angle of the cable employed to apply force to the tong arm and which will be inexpensive to manufacture and simple and reliable in operation, involving few moving parts.